Fine-scale horizontal distributions of multiple species of larval tuna off the Nansei Islands, Japan

To quantitatively evaluate the distribution of tuna larvae relative to oceanographic conditions, we conducted investigations off the Nansei Islands in the western North Pacific in June from 2015 to 2017. Five species, namely Pacific bluefin tuna Thunnus orientalis (PBF), yellowfin tuna T. albacares (YFT), skipjack tuna Katsuwonus pelamis (SKJ), frigate tuna Auxis thazard, and bullet tuna A. rochei (BT), were collected in each year. The most dominant species was BT throughout the 3 yr period, followed by SKJ in 2015 and YFT in 2016 and 2017. The horizontal larval distributions of the 5 species were largely influenced by the Kuroshio Current: larvae of the 2 Auxis species were distributed in the Kuroshio and the Kuroshio inshore waters, whereas those of the other species were found in the Kuroshio offshore waters. These differences are consistent with the differences in spawner distributions among the tunas. Generalized additive models (GAMs) indicated that the larval densities were affected by the sea surface height anomaly and that the larvae were not always amassed by horizontal transport. Sea surface temperature (SST) and salinity possibly influenced the larval physiology and survival, thereby determining their densities. In the GAMs, PBF and YFT showed similar responses to SST, and YFT and SKJ similarly responded to salinity. To avoid overlapping their ecological niches, the larvae of 3 species (PBF, YFT, and SKJ) are expected to differ in other ways, including their vertical distributions and feeding habits.